The invention relates to a transmitter of radioelectric signals comprising a plurality of amplifiers, each amplifier being optimized to amplify a signal whose frequency is included in a predetermined frequency band, said signal alternately carrying information and being in the quiescent state during first and second predetermined periods of time.
An apparatus of this type, for example a radiotelephone, is capable of interacting with a plurality of communication networks, each one of these networks being designed to route signals whose frequency is in a frequency band that is generally predetermined by a standard.
The GSM standard, for example, includes a frequency band ranging from 880 to 915 MHz, while the DCS1800 standard includes a frequency band ranging from 1710 to 1795 MHz. Other standards, such as inter alia AMPS (acronym of Advanced Mobile Phone System) or PCS (acronym of Personal Communication System) include frequency bands that are centered around 800 and 190 MHz, respectively.
A transmitter capable of interacting with several networks, commonly referred to as multistandard, thus generates, dependent upon the network with which it is to communicate, a radio signal of adequate frequency, i.e. a frequency included in the frequency band covered by the corresponding standard. This radio signal is amplified by a power amplifier before it is transmitted by means of an antenna system.
It is known that an amplifier operates optimally in a given frequency band; for this reason, a transmitter capable of communicating with a plurality of communication networks generally comprises a number of power amplifiers, arranged in parallel upstream of the antenna system, which is equal to the number of networks capable of interacting with said apparatus.
When the apparatus is in the operating mode, the power amplifiers all receive the radio signal, but only one of said power amplifiers can supply an optimally amplified signal in the frequency band of the radio signal. Therefore, it is necessary to inhibit all the other power amplifiers. This is generally achieved by applying a negative voltage to the inputs of the amplifiers to be deactivated, thus enabling a permanent selection of the most suitable power amplifier, the negative voltages being applied throughout a period of time during which the apparatus must use a given communication network.
The known multistandard transmitters must thus comprise means for generating a continuous negative voltage. Two solutions are available for this purpose. A first solution consists in amplifying and rectifying a carrier of the radio signal. This solution, which is employed, for example, in the circuits MRFICO919 and MRFIC1819, marketed by Motorola, requires a complex and expensive circuitry, leading to a substantial increase of the cost price of the transmitter.
A second solution consists in employing a DC-DC converter, which is used as a negative voltage generator, as in the case of, for example, the Motorola circuits MC33169 and MC33170. However, such converters are noisy and require filtering of the signals they supply in order to preclude that said signals introduce parasitic rays into the radio signal. This type of filtering also requires the application of complex and expensive structures.
It is an object of the invention to overcome these drawbacks by providing a transmitter wherein the power amplifiers which are not optimized for the frequency band including the frequency of the radio signal can be inhibited without the generation of a continuous negative voltage.
In accordance with the invention, a transmitter in accordance with the opening paragraph comprises:
detection means intended to supply a detection signal having active and inactive states during the first and the second predetermined periods of time, respectively, and
control means intended to supply inhibition signals, when the detection signal is in the active state, which inhibition signals are intended to render those amplifiers inactive which are not optimized to execute an amplification in the frequency band wherein the frequency of the signal to be amplified is included.
In such a transmitter, the selection of the amplifier that is optimized to amplify the radio signal is not made permanently, but by means of pulses, i.e. only when the signal to be amplified effectively carries information. This means that it is not necessary to use a continuous negative voltage to inhibit the other amplifiers; instead a succession of pulses is used, which can be more readily generated, for example on the basis of the radio signal itself.
In one of the embodiments, the invention also relates to a generator of negative voltage pulses comprising an output terminal intended to produce said pulses, and an input terminal intended to receive a command signal intended to be periodically in an active state for a period of time that defines the duration of the pulses, which generator comprises a capacitive element, one terminal of which is connected to the output terminal of the generator, and another terminal of which is connected to first and second supply terminals via, respectively, a resistor and a main current path of a first transistor, the conduction of said first transistor being regulated by means of the command signal.
The invention more generally relates to a method of selecting an amplifier from a plurality of amplifiers, each one of said amplifiers being optimized to amplify a signal whose frequency is included in a predetermined frequency band with a view to amplifying a signal that alternates between carrying information and being in the quiescent state during first and second predetermined periods of time, which method includes an inhibition step, during the first period of time, thereby inhibiting those amplifiers which are not optimized to execute an amplification in the frequency band wherein the frequency of the signal to be amplified is included.
In an embodiment of this method, an amplifier will be advantageously inhibited by applying a negative voltage pulse to an input terminal of said amplifier, the duration of said pulse being equal to the predetermined first period of time.